According to the Britannica Concise Encyclopedia, a range finder is an instrument used to measure the distance from the instrument to a selected point or object. The optical range finder, used chiefly in cameras, consists of an arrangement of lenses and prisms set at each end of a tube. The object's range is determined by measuring the angles formed by a line of sight at each end of the tube; the smaller the angles, the greater the distance, and vice versa. Since the mid-1940s, radar has replaced optical range finders for most military targeting, and the laser range finder, developed in 1965, has largely replaced optical range finders for surveying and radar in certain military applications.
The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Lasers have the property of efficiently collimating their beam and therefore concentrate their energy in a narrow angular range. The Field-of-illumination (FOI) of the laser rangefinder is therefore much narrower than the Field-of-view (FOV) of the detector. When the pulse is reflected off the target, only a portion of the reflected light is reflected toward the detector. This portion is what is captured by the laser rangefinder and a detection of an object is only identified if a reflection threshold is reached at the detector.
The concentrated laser beam has many advantages. If the surface of the object reflects a great portion of the laser beam it receives toward the detector, it is very efficient. However, if the surface of the object is not exactly at a right angle to the beam and it has some specular reflection effects, the major portion of the reflection will be oriented in a direction other than that of the detector. The object will then not be detected. Additionally, if some sections of the surface of an object illuminated by the laser beam have very poor reflectivity, the amplitude of the reflection received by the detector will not be enough to reach the preset threshold and the object will not be detected. The laser rangefinder therefore presents reliability issues.
A car is a good example of an object which has surfaces with varying reflectivity and portions with different shapes and angles. Some sections of the car which have very high reflectivity, such as the retro-reflector typically located at the back of the vehicles will have extremely high reflectivity toward the source but are very small in size and unlikely to receive the very collimated beam of the laser source. Some sections of the car with varying shapes will reflect the very collimated beam of the laser source in many different directions and not necessarily toward the detector and may therefore go undetected.
There are therefore some drawbacks to the use of prior art systems, especially laser rangefinders.